1. Field of the Invention
The present invention relates to a transfer device that transfers a toner image formed on an image-carrier rotating member onto transfer medium via an intermediate transfer belt.
2. Description of the Related Art
Conventionally, there have been proposed quadruple-tandem type color image forming apparatuses. In such a color image forming apparatus, four photosensitive drums, each of which works as an image-carrier rotating member for carrying a toner image in a specific color corresponding to each of black, cyan, magenta, and yellow, are arranged in a row on a single plane. The toner images formed on the photosensitive drums by a developing unit are transferred, as superimposed on one another (by using an offset method), onto a surface of an intermediate transfer belt by a primary transfer roller corresponding to each of the photosensitive drums. The toner image obtained as a result of the superimposing process is secondarily transferred onto a transfer medium by a secondary transfer roller. Thereafter, the toner image on the transfer medium is fixed by applying a heat and a pressure with a fixing device. Thereby, a color image is obtained.
In such a color image forming apparatus having a single-color mode (for example, monochrome mode) and a full-color mode, to protect the photosensitive drums and the intermediate transfer belt from mechanical tear and wear as much as possible, when the monochrome mode is selected, three primary transfer rollers corresponding to the cyan, magenta, and yellow photosensitive drums, that is, the photosensitive drums other than the black photosensitive drum used at the monochrome mode are detached from the intermediate transfer belt by an attaching/detaching mechanism. In the attaching/detaching mechanism, for example, an end of a linking member pivots on an axis so that the linking member swings around the pivot axis as a supporting point, and the primary transfer roller is arranged on the linking member.
However, when all the photosensitive drums including the photosensitive drum used in the monochrome mode are arranged in a row on a single plane, a distance required for detaching the three primary transfer rollers from the intermediate transfer belt (or a distance required for attaching the three primary transfer rollers to the intermediate transfer belt when the full-color mode is set) becomes large.
The above problem is explained in detail below with reference to FIG. 7. The primary transfer roller C2 for cyan that is positioned in the third place from the most upstream along a moving direction of the intermediate transfer belt (i.e., in a direction of an arrow shown in FIG. 7) moves distance E to detach from the intermediate transfer belt. However, a primary transfer roller Y2 for yellow that is positioned in the most upstream moves distance E′, which is approximately three times as large as distance E. This is why it takes a long time until the image forming apparatus becomes ready to transfer, when the image forming apparatus is switched between the monochrome mode and the full-color mode, and it is difficult to reduce a size of a driving motor that drive an attaching/detaching mechanism for attaching or detaching the primary transfer rollers to or from the intermediate transfer belt.
To solve the problems described above, Japanese Patent Application Laid-open No. 2002-62776 discloses a transfer device in which, as shown in FIG. 8, a photosensitive drum Bk1 for black used in the monochrome mode is positioned closer to the intermediate transfer belt than the other three photosensitive drums C1, M1, and Y1 used in the full-color mode are. In other words, a rotation axis of the photosensitive drum Bk1 for black (on plane B indicated by a solid line shown in FIG. 8) is positioned closer to the intermediate transfer belt and displaced from a plan on which rotation axes of the other photosensitive drums C1, M1, and Y1 for the color use fall (plan A indicated by a dashed line shown in FIG. 8). According to the above technique, the largest distance required for detaching the primary transfer rollers from the intermediate transfer belt is distance E″, which is significantly smaller than distance E′ shown in FIG. 6.
However, the technique disclosed in Japanese Patent Application Laid-open No. 2002-62776 still has a problem where a width of a portion of the intermediate transfer belt in contact with the photosensitive drum used in the monochrome mode differs depending on whether the primary transfer rollers corresponding to the three photosensitive drums used at the color mode are detached from the intermediate transfer belt. More specifically, as shown in FIG. 9, when the full-color mode is selected, that is, when the primary transfer rollers C2, M2, and Y2 are in contact with the intermediate transfer belt and are opposite to the photosensitive drums C1, M1, and Y1 used at the full-color mode, a contact angle that indicates the width of the portion of the photosensitive drum Bk1 for black in contact with the intermediate transfer belt is angle θ1 as shown with a solid line in FIG. 9. When the monochrome mode is selected, and the primary transfer rollers C2, M2, and Y2 opposite to the photosensitive drums C1, M1, and Y1 used in the full-color mode are detached from the intermediate transfer belt, the contact angle is angle θ2 as shown with a broken line in FIG. 9. It means that, the width of the portion of the intermediate transfer belt in contact with the photosensitive drum Bk1 changes (θ1>θ2). As a result, because the conditions under which an image is transferred from the photosensitive drum Bk1 set to be used in the monochrome mode onto the intermediate transfer belt differs depending on which mode is selected, the quality of the image also varies.